During the drilling, completion, and production of oil and gas wells, it is often necessary to engage in ancillary operations, such as monitoring the operability of equipment used during the drilling process or evaluating the production capabilities of formations intersected by the wellbore. For example, after a well or well interval has been drilled, zones of interest are often tested to determine various formation properties such as permeability, fluid type, fluid quality, formation pressure, and formation pressure gradient. These tests determine whether commercial exploitation of the intersected formations is viable.
Other formation tests include seismic exploration operations. The field of seismology focuses on the use of artificially generated elastic waves to locate mineral deposits such as hydrocarbons, ores, water, and geothermal reservoirs. Exploration seismology provides data that, when used in conjunction with other available geophysical, borehole, and geological data, can provide information about the structure and distribution of rock types and contents.
Seismic exploration generally involves a seismic wave source creating and imparting vibrational energy into the ground. That energy may be in the form of a relatively short duration, large amplitude “impulse”, or longer duration, lower amplitude vibrations. The energy imparted to the ground propagates generally downward and reflects off various subsurface structures, such as interfaces between different rock formations. The reflected energy waves are then detected by receivers called “geophones”. The term “geophone” is used throughout this disclosure to refer to any type of seismic detector. The receivers may be located either on a drill string, a wireline tool, the production casing, or even in the annulus between the casing and the borehole wall. The receivers may also be deployed in a large array on the surface of the earth.
When the waves are detected by a receiver, the geophone records a “seismogram”. Seismograms are commonly recorded as digital samples representing the amplitude of a received seismic signal as a function of time. With multiple geophones, the digital samples can be formed into a 3-dimensional array with each sample in the array representing the amplitude of the seismic signal as a function of time (t) and position on the earth (x,y).
The collection of seismic samples as a function of time (t) for one position in the earth is referred to as a “seismic trace.” The collection of seismic traces forming an array are commonly referred to as “seismic data volumes.” A seismic data volume depicts the subsurface layering of a portion of the earth. It is the principal tool that a geophysicist uses to determine the nature of the earth's subsurface formations. The seismic data volume can be studied either by plotting it on paper or displaying it on a computer monitor. A geophysicist can then interpret the information. When displaying the seismic data volume along a principle direction, crosslines, inlines, time slices, or horizon slices can be made. The seismic data volume can be mathematically processed in accordance with known techniques to make subtle features in the seismic data more discernible. The results of these processing techniques are known as seismic “attributes.” The images may also be compared over a period of time to follow the evolution of the subsurface formation over time.
An example of a conventional method of seismic exploration referred to as VSP consists in coupling the wall of a well to a string of seismic receivers placed at intervals along a conventional logging cable in order to pick up the waves reflected by the discontinuities of the surrounding formation in response to the waves emitted by a source outside the well. Seismic sources are then installed with different offsets and/or azimuths in relation to the well for creating the seismic waves.
Onshore, the sources can be readily placed in contact with the ground or at the bottom of a cased hole deep enough for the source to be in contact with the formation below the weathered layer. Several independent sources coupled with the ground with different offsets in relation to the well can be successively activated for each position of the VSP logging tool.
Offshore, in order to carry out similar multi-offset, multi-azimuth VSP type exploration sessions, a boat is used to tow an immersed impulsive source (air, water or explosive gun, marine vibrator, etc.) through a zone around the well to a succession of “shooting” positions and to carry out, in each one of them, a succession of emission-reception cycles. Several workboats, each towing a marine source or multiple sources, may also be used to trigger the sources in sequence for each position of the logging tool.